The cytochrome P450s metabolize a wide variety of xenobiotic and endogenous compounds. Biochemical, biophysical and computational approaches were applied to examine the structure-function relationships which govern the interactions of P450s with substrates and microsomal proteins. The kinetics of CO binding to P450 was used as a probe of P450 conformation and dynamics, to define the effect of various drugs and carcinogens on P450s. Of particular interest is the finding that human P450s 3A4 and 1A1, which respectively metabolize a variety of drugs and carcinogens, can assume alternate conformations. In contrast, the alcohol-inducible and carcinogen-metabolizing P450 2E1, which metabolizes a more narrow range of substrates, is more conformationally homogeneous. Ethanol was found to stabilize this P450 and modulate its substrate interactions. We employed molecular modeling to generate homology models of rat 2B1 and human P450 1A2. Peptide mimics of the P450 2B1 surface were prepares that disrupt the P450 interaction with its associated reductase. The active site structures of both P450s were consistent with their specificities toward known substrates or inhibitors. In conjunction with computational screening of a small molecule structure database, we are currently employing the P450 1A2 model to discover new inhibitors for this drug and carcinogen-metabolizing P450. In conjunction with previous work, our P450 homology models are consistent with P450-protein interactions as well as substrate specificity.